This feature was discussed in #6123, but it doesn't look like anything was ever incorporated into the IPython Notebook.

Here's a brief overview of the changes:

- Display of messages from other clients can be toggled on and off from within a notebook, either using the ``<M-m>e`` keyboard shortcut in the web UI, or through the option in the "Kernel" menu.
- notebook.js controls whether messages are displayed through a callback that is invoked from kernel.js when no callbacks are available for a message.
- The UI displays ``execute_input`` messages originating from an other clients in new cells at the end of the notebook. Output messages (``execute_result`` et al.) will only be displayed if a cell exists with a matching message ID.

Pending design questions:

- Should each ``execute_input`` message cause a new cell to be created?
- Should new cells be placed at the end of the notebook, or elsewhere? If the latter, what criteria should be followed?

IPython/html/static/notebook/js/actions.js

@@ -253,6 +253,14 @@ define(['require'
                 env.notebook.delete_cell();
             }
         },
+        'toggle-unsolicited-message-display':{
+            help: 'toggle display from external clients',
+            icon: 'fa-sitemap',
+            help_index: 'gb',
+            handler: function (env) {
+                env.notebook.toggle_ignore_unsolicited_msgs();
+            }
+        },
         'interrupt-kernel':{
             icon: 'fa-stop',
             help_index : 'ha',

IPython/html/static/notebook/js/keyboardmanager.js

@@ -95,6 +95,7 @@ define([
             'space' : 'ipython.scroll-down',
             'down' : 'ipython.select-next-cell',
             'i,i' : 'ipython.interrupt-kernel',
+            'e': 'ipython.toggle-unsolicited-message-display',
             '0,0' : 'ipython.restart-kernel',
             'd,d' : 'ipython.delete-cell',
             'esc': 'ipython.close-pager',

IPython/html/static/notebook/js/menubar.js

@@ -278,6 +278,9 @@ define([
         });
         
         // Kernel
+        this.element.find('#toggle_unsolicited').click(function() {
+            IPython.notebook.toggle_ignore_unsolicited_msgs();
+        });
         this.element.find('#int_kernel').click(function () {
             that.notebook.kernel.interrupt();
         });

IPython/html/static/notebook/js/notebook.js

@@ -135,6 +135,7 @@ define([
         this.undelete_below = false;
         this.paste_enabled = false;
         this.writable = false;
+        this.ignore_unsolicited_msgs = false;
         // It is important to start out in command mode to match the intial mode
         // of the KeyboardManager.
         this.mode = 'command';
@@ -1562,6 +1563,60 @@ define([
         this.get_selected_cell().toggle_line_numbers();
     };
     
+    // Support for displaying input and output messages from other iPy clients.
+
+    /**
+     * Toggles the ability to display input/output message events from
+     * externally connected clients (i.e. other iPython shells, vim-ipython,
+     * etc).
+     *
+     * @method toggle_ignore_unsolicited_msgs
+     */
+    Notebook.prototype.toggle_ignore_unsolicited_msgs = function () {
+        this.ignore_unsolicited_msgs = !this.ignore_unsolicited_msgs;
+        this.events.trigger('toggle_unsolicited_msgs.Notebook',
+                [this.ignore_unsolicited_msgs]);
+        return this.ignore_unsolicited_msgs;
+    };
+
+    /**
+     * Handles the display of unsolicited messages, i.e. inputs or outputs that
+     * were generated by a client other than this notebook. New messages are
+     * displayed at the bottom of the notebook.
+     *
+     * @method handle_unsolicited_msg
+     */
+    Notebook.prototype.handle_unsolicited_msg = function(msg) {
+        if (this.ignore_unsolicited_msgs) {
+            return;
+        }
+        if (msg.msg_type == 'execute_input') {
+            var cell = this.insert_cell_at_bottom('code');
+            if (cell) {
+                var cell_index = this.ncells() - 1;
+                cell.last_msg_id = msg.parent_header.msg_id;
+                cell.set_text(msg.content.code);
+                cell._handle_execute_reply(msg);
+                this.scroll_to_cell(cell_index);
+                this.select(cell_index);
+            }
+        } else {
+            /* Find the input cell that corresponds with the output, then add
+             * the contents to the cell's output area.
+             */
+            var count = this.ncells();
+            while (count--) {
+                var cell = this.get_cell(count);
+                if (cell && cell.last_msg_id == msg.parent_header.msg_id) {
+                    cell.output_area.handle_output(msg);
+                    this.scroll_to_cell(count);
+                    this.select(count);
+                    break;
+                }
+            }
+        }
+    };
+
     /**
      * Set the codemirror mode for all code cells, including the default for
      * new code cells.
@@ -1639,6 +1694,7 @@ define([
                 cell.set_kernel(this.session.kernel);
             }
         }
+        this.kernel.unsolicited_msg_callback = $.proxy(this.handle_unsolicited_msg, this);
     };
     Notebook.prototype._session_start_failed = function (jqxhr, status, error){
         this._session_starting = false;

IPython/html/static/notebook/js/notificationarea.js

@@ -279,6 +279,11 @@ define([
             nnw.warning(error.message || "Notebook copy failed");
         });
         
+        this.events.on('toggle_unsolicited_msgs.Notebook', function(evt, ignored) {
+            var msg = (ignored? "Ignoring": "Showing") + " I/O from external clients";
+            nnw.set_message(msg, 1000);
+        });
+
         // Checkpoint events
         this.events.on('checkpoint_created.Notebook', function (evt, data) {
             var msg = "Checkpoint created";

IPython/html/static/services/kernels/kernel.js

@@ -47,6 +47,7 @@ define([
         this.session_id = utils.uuid();
         this._msg_callbacks = {};
         this.info_reply = {}; // kernel_info_reply stored here after starting
+        this.unsolicited_msg_callback = null;
 
         if (typeof(WebSocket) !== 'undefined') {
             this.WebSocket = WebSocket;
@@ -136,6 +137,7 @@ define([
         this._iopub_handlers = {};
         this.register_iopub_handler('status', $.proxy(this._handle_status_message, this));
         this.register_iopub_handler('clear_output', $.proxy(this._handle_clear_output, this));
+        this.register_iopub_handler('execute_input', $.proxy(this._handle_input_message, this));
         
         for (var i=0; i < output_msg_types.length; i++) {
             this.register_iopub_handler(output_msg_types[i], $.proxy(this._handle_output_message, this));
@@ -994,6 +996,11 @@ define([
     Kernel.prototype._handle_output_message = function (msg) {
         var callbacks = this.get_callbacks_for_msg(msg.parent_header.msg_id);
         if (!callbacks || !callbacks.iopub) {
+            if (this.unsolicited_msg_callback) {
+                // The message came from another client. Let the UI decide what
+                // to do with it.
+                this.unsolicited_msg_callback(msg);
+            }
             return;
         }
         var callback = callbacks.iopub.output;
@@ -1002,6 +1009,20 @@ define([
         }
     };
 
+    /**
+     * Handle an input message (execute_input).
+     *
+     * @function _handle_input message
+     */
+    Kernel.prototype._handle_input_message = function (msg) {
+        var callbacks = this.get_callbacks_for_msg(msg.parent_header.msg_id);
+        if (!callbacks && this.unsolicited_msg_callback) {
+            // The message came from another client. Let the UI decide what to
+            // do with it.
+            this.unsolicited_msg_callback(msg);
+        }
+    };
+
     /**
      * Dispatch IOPub messages to respective handlers. Each message
      * type should have a handler.

IPython/html/templates/notebook.html

@@ -228,6 +228,9 @@ class="notebook_app"
                 </li>
                 <li class="dropdown"><a href="#" class="dropdown-toggle" data-toggle="dropdown">Kernel</a>
                     <ul id="kernel_menu" class="dropdown-menu">
+                        <li id="toggle_unsolicited"
+                            title="Toggle display of unsolicited messages.">
+                            <a href="#">Show/ignore unsolicited messages.</a></li>
                         <li id="int_kernel"
                             title="Send KeyboardInterrupt (CTRL-C) to the Kernel">
                             <a href="#">Interrupt</a>